The present invention relates to the reduction of noise caused by the movement of aircraft rotor blades such as those found in helicopters. More particularly, the present invention is concerned with reducing high speed impulsive noise in aircraft rotor blades as well as the additional problem of noise caused by blade vortex interaction. It is desirable to reduce noise of these types from the human factors standpoint of reducing environmental noise pollution and possible damage to hearing. In addition, in surveillance operations or military combat situations, such noise impairs the ability to avoid detection by criminal suspects or hostile forces.
The primary sources of helicopter blade tonal noise are thickness noise, loading noise, blade vortex interaction noise (BVI) and high-speed impulsive noise (HSI). In addition to these tonal sources there are a wide variety of broad band noise sources, all of which are produced by random pressure fluctuations on the rotor blade surfaces. In hover, thickness noise and loading noise are the predominant sources. Thickness noise comes about because the blade displaces air as it rotates. The steady lift and drag forces on the blade generate loading noise. Broad band noise is generally less intense than tonal noise, and, of the tonal noise sources, BVI and HSI noise are the most intense and annoying when they occur.
HSI noise is related to thickness noise. HSI noise occurs when the forward speed of the blade tip relative to the air approaches sonic velocity. This typically only occurs on the side of the helicopter where the blade is moving in the same direction as the helicopter. As the air accelerates to move around the finite thickness of the blade shocks form that lead to a dramatic increase in impulsive noise. The result can be considered as two steady volume sources on the blade, an outward flowing source on the front half and an inward flowing source on the rear half of the blade. The fact that the steady sources are in motion produces sound. In forward flight the advancing blade may have regions where the flow is locally supersonic because of the finite thickness of the blade, even though the sum of the blade's forward velocity and the helicopter flight speed may be subsonic. These local regions of supersonic flow can lead to significant intensification of the sound that is typically referred to as HSI noise which occurs at high forward speed when the blade tip mach number on the advancing side approaches 0.85. The intensification of the noise appears to be associated with the appearance of shock near the blade tip where the flow becomes locally sonic. As already noted, HSI noise has many of the characteristics of thickness noise in that it radiates strongly in the plane of the rotor. However, HSI noise is considerably more intense than thickness noise and the resulting pressure pulses are more asymmetric in nature due to nonlinear effects.
The approach to reducing high speed impulsive noise in rotor blades has been to employ thin, tapered, or swept end blades. While these modifications can reduce HSI noise, they can have deleterious effects such as stall at lower angles of attack and unfavorable pitching--moment characteristics.
BVI noise occurs when a blade encounters the trailing edge vortex of a preceding blade, typically during low speed forward descending flight. The variations in the flow velocity in the vortex on interacting with the blade cause pressure fluctuations that radiate and create intense impulsive noise. The intensity of the noise depends on the strength of the vortex and the closeness of its approach to the helicopter blade. Consequently, modifying the velocity distribution in the vortex is a method by which BVI noise can be controlled. One approach for affecting the velocity distribution in the vortex is described in U.S. Pat. No. 5,791,875 to Ngo which describes a system for directing pressurized air out of the rotor blade tip to reduce blade vortex interaction. The Ngo approach, however, does not take into account all of the factors that contribute to the production of BVI noise, nor does it provide for the reduction of HSI noise. Furthermore, the Ngo approach of cancelling the blade tip vortices by injecting air at the tip can be counterproductive in that it can kill the lift of the blade, thus reducing helicopter performance.
Accordingly, there is a need for a system for reducing aircraft rotor noise, and, in particular, for reducing high speed impulsive noise, especially in combination with a system for also reducing blade vortex interaction noise.